Identification of novel immunoreactive proteins of Streptococcus zooepidemicus with potential as vaccine components

Streptococcus zooepidemicus is an important opportunistic pathogen of the equine respiratory and reproductive tracts. A normal tonsillar and mucosal commensal, it becomes invasive under conditions of stress such as virus infection, weaning, high temperature, prolonged transportation and failure of uterine involution. The aim of this study was to evaluate the vaccine potential of several surface exposed and secreted proteins of a novel mucoid clone of SzNC78 (ST-307) from an epizootic of equine respiratory disease.

An expression gene library of SzNC78 was probed with a pool of convalescent equine sera from a clonal epizootic of respiratory disease. Eleven proteins were selected and purified based on putative function, surface expression or secretion and possible importance as virulence factors. Three additional proteins (AhpC, GAPDH and enolase) were also included based on their putative virulence function. Groups of ICR mice were vaccinated subcutaneously with each recombinant antigen and QuilA and later challenged with SzNC78.

SzM protected 100% mice (P<0.01), followed by SzP and HylC which protected 90% mice (P=0.01). MAP, SzMAC and ScpC each protected 63% (P<0.05) mice. No control mouse survived challenge. SzM, MAP and ScpC in combination protected mice against a 10 fold higher dosage of SzNC78 than each antigen given separately. Protection against heterologous challenge (SzW60) was conferred by combinations of HylC+ScpC (60%; P=0.05), followed by HylC+MAP (50%; P=0.06) and ScpC+MAP (40%; P=0.1). Serum antibody responses of horses recently recovered from Sz respiratory infection were highest against ScpC, MAP and SzP. A combination of SKC and Sz115 stimulated no protection against challenge with SzW60.

A subset of Sz proteins reactive with convalescent equine antibody have potential as components of experimental vaccines to aid in prevention of opportunistic Sz infections.